Mongo Context

All models must contain an implicit salat Context. The context is somewhat like a hibernate dialect.
You can override mapping names and configure how salat does it's type hinting. read more about it here

In the sample there is a custom mongoContext, partly because we need to add plays classloader to salat so it knows when to reload it's graters,
but also so we can override all models id fields to be serialized to MongoDB's _id.